1. Field of the Invention
This invention relates to a method and apparatus for coating materials by ion plating.
2. Prior Art
The following process has become known as ion plating: a vapor of an evaporation material from a vapor source and a gas introduced from a gas inlet are ionized by an ionizing means, and a film of a metal, alloy, compound, etc. is deposited on a substrate surface which is at a negative potential relative to that of the vapor source. Especially, the process where the ionized vapor of an evaporation material reacts with an ionized gas atmosphere to form a compound is called reactive ion plating.
The technology for producing a gold-colored titanium nitride (TiN) layer onto the surface of a decorative part by this process has been disclosed, for example, in U.S. Pat. No. 4,226,082. In the process of ion plating, a negatively biased substrate surface is always subjected to positive ion bombardment and simultaneously deposited with the coating material. Accordingly, poorly adhesive particles are bombarded and evaporated again and only highly adhesive particles are deposited. As a result, a coating having excellent adherence and close packing can be produced, but the deposition rate is slow, when compared with usual evaporations.
The close-packed coating hardly relaxes a stress and increases the residual stress as the coating grows. This stress will cause destructive cracking. It is, therefore, difficult to grow a film to an appropriate thickness.
It has been proposed, therefore, to use ion plating at the initial stage of a coating process which requires adhesion and then to exchange it to a usual evaporation process so as to grow the coating (see Japanese Published Unexamined Patent Application No. 50-133936).
However, usual evaporations have no means to ionize the vapor of an evaporation material. Therefore, reactivity is not full enough to produce a coating of intended quality, especially in the formation of compound films. Furthermore, usual evaporations are performed under relatively high vacuum conditions, thus causing less vapor to disperse roundabout. It is difficult to coat a film with a uniform thickness onto the sides and back of a substrate of intricate shape.